The present invention relates generally to extractive processes using aqueous solutions of alkali metal hydroxides to recover phenol from organic streams. More particularly, the present invention relates to a method for determining the quantity of phenol that has been extracted into an aqueous solution of an alkali metal hydroxide. The invention also relates to the optimization of an extractive process for the recovery of phenol from various distillation streams by monitoring the pH of an aqueous extraction solvent containing an alkali metal hydroxide.
For a number of years, it has been common practice in phenol manufacturing facilities to use aqueous solutions of alkali metal hydroxides to recover phenol from organic streams, such as distillation overheads or bottoms, by extraction into the aqueous stream. This method operates by reaction of the phenol with the alkali metal hydroxide to form a water soluble alkali metal phenate, such as sodium phenate. The basic process by which phenol is converted into sodium phenate is shown by the equation;
1 PhOH+1 NaOHxe2x86x921 PhOxe2x88x92Na++1 H2O 
As the free sodium hydroxide is consumed by conversion to sodium phenate, the pH of the aqueous solution decreases.
Historically, alkali metal hydroxide solutions have been used to extract phenol from an organic stream in an equimolar proportion to the amount of alkali metal hydroxide present in the aqueous stream. That is, as in the example shown, one mole of sodium hydroxide reacts with one mole of phenol to produce one mole of sodium phenate, which is extracted into the aqueous stream.
It is disclosed in U.S. Pat. Nos. 3,277,185 and 4,973,766 that solutions of alkali metal phenate can act as a solvent for free phenol, causing the phenol to have a greater affinity for the aqueous stream. Thus, it is possible to extract greater than an equimolar amount of phenol into an aqueous stream from an organic stream. This phenol is extracted into the aqueous stream as free solubilized phenol, rather than phenate. U.S. Pat. No. 3,277,185 discloses using an aqueous solution of alkali metal phenate at a concentration of 20 to 40% as an extraction solvent to remove phenol from neutral oils in a process for the gasification of solid fuels. U.S. Pat. No. 4,973,766 discloses the use of aqueous solutions of an alkali metal phenate at concentrations of 5 to 15% to extract phenol from a rectification purge containing 20 to 95% phenol.
The goal of a phenol recovery unit is to remove the maximum amount of phenol possible from the organic stream being treated at minimum cost. It is therefore desirable to take advantage of the ability of solutions of alkali metal phenate to absorb free phenol, in a phenol recovery system that uses aqueous solutions of alkali metal hydroxides as an extracting solvent. By doing so the quantity of alkali metal hydroxide used in phenol recovery processes can be significantly reduced. However, since free phenol is not infinitely soluble in aqueous solutions of alkali metal phenate, an underage of alkali metal hydroxide would result in undesirable loss of phenol to the organic effluent. Also, since the organic streams treated in this fashion often have value themselves, it is desirable to produce an organic effluent stream that is essentially free of phenol. Therefore, in order to ensure cost efficiency and maximum removal of phenol from the organic stream, care must be taken to ensure that an appropriate quantity of aqueous alkali metal hydroxide is used in the recovery process.
Accordingly, it is desirable to provide a method for determining the quantity of phenol that has been extracted into an aqueous solution of an alkali metal hydroxide. Further, it is desirable to provide a method for regulating the quantity of aqueous alkali metal hydroxide used in phenol recovery operations to ensure maximum recovery of phenol, acceptable organic effluent and cost effectiveness.
It is an object of the present invention to provide a method for determining the quantity of phenol that has been extracted into an aqueous solution of an alkali metal hydroxide.
It is another object of the present invention to provide a method for optimizing the use of an aqueous solution of an alkali metal hydroxide in a process for recovering phenol from an organic stream, based on the pH of the aqueous stream at, at least, one point in the system.
It is a further object of the present invention to provide an apparatus for optimizing the use of an aqueous solution of an alkali metal hydroxide in a process for recovering phenol from an organic stream, based on the pH of the aqueous stream at, at least, one point in the system.
In accordance with one embodiment of the invention, a method for indirectly determining the quantity of phenol that has been extracted into an aqueous solution of an alkali metal hydroxide is provided. In this method, aqueous solutions of an alkali metal hydroxide of known concentration, e.g. 20% sodium hydroxide, are titrated with phenol. The pH data collected is used to develop a curve showing solution pH relative to phenol concentration. The curve thus developed is used to determine phenol concentrations in aqueous solutions of alkali metal hydroxides by simply measuring pH.
In accordance with another embodiment of the present invention, a method is provided for regulating the rate at which an aqueous solution of alkali metal hydroxide is fed to a phenol recovery unit as an extracting solvent in counter-current flow to a phenol containing organic stream. The pH of the aqueous stream is monitored at, at least one point, in the phenol recovery unit. Based on the pH of the aqueous stream, it is possible to determine the concentration of phenol in the aqueous stream and if any free phenol is present in the aqueous stream. The flow of aqueous alkali metal hydroxide to the unit can then be held constant, increased or decreased accordingly to optimize the recovery of phenol and usage of aqueous extracting solvent. The phenol containing organic stream preferably comprises either cumene or xcex1-methyl styrene.
In a further embodiment of the current invention, an apparatus is provided, which makes use of the inventive methods herein disclosed. The apparatus is a unit, which uses an aqueous solution of an alkali metal hydroxide for recovering phenol from an organic stream, such as distillation overheads or bottoms. The feed rate of the alkali metal hydroxide to the unit is regulated by measuring the pH at, at least, one point in the unit and adjusting the feed rate accordingly.
There has thus been outlined, rather broadly, the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described below and which will form the subject matter of the claims appended hereto.
In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract included below, are for the purpose of description and should not be regarded as limiting.
As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention.